1. Field of the Invention
The present invention relates to a transfer apparatus for transferring a toner image carried on a photosensitive body onto a recording medium in an image forming apparatus that forms an electrophotographic image, and more particularly to a transfer apparatus employing a transfer roller having a dielectric layer formed on its outer surface.
2. Description of the Related Art
There are known some types of an image forming apparatus for forming an electrophotographic image, employing a transfer apparatus equipped with a semiconductor transfer roller to enhance transfer efficiency during the transfer process. The transfer apparatus equipped with the transfer roller nips a recording medium at a transfer portion having a predetermined nip width and presses the transfer roller against the surface of a photosensitive body, and applies a transfer voltage to the transfer roller while the recording medium passes through a space between the photosensitive body and transfer roller, whereby toner carried on the surface of the photosensitive body is transferred onto the surface of the recording medium.
As such a semiconductor transfer roller is known a roller composed of a cylindrical conductive substrate and a conductive elastic rubber of high resistance with which a surface of the substrate is covered.
However, a resistance value of an elastic material of high resistance varies considerably, and an error in resistance value can occur among different transfer rollers and among different portions in a single transfer roller. Also, a resistance value varies with an external environment such as humidity.
Hence, in order to stabilize a resistance value, a dielectric transfer roller having a resistance layer and a dielectric layer formed on the outer surface of the semiconductor substrate in this order has been proposed as a transfer roller of the conventional transfer apparatus.
Japanese Unexamined Patent Publication JP-A 1-230079 (1989) discloses a monocomponent developing apparatus using a developing roller having at least a dielectric layer deposited on the upper surface of a supporting layer made of an elastic material, wherein, based on a saturation development model of a capacitor type, the resistance of the supporting layer is set lower than a value determined by a bias potential, an amount of charge of toner, etc.
Japanese Unexamined Patent Publication JP-A 3-87759 (1991) discloses a developing method of readily obtaining a sharp and uniform image of high density without any fog on the background by adjusting the following: an amount of charge of toner adhering to a surface holding an electrostatic latent image as a result of development; an amount:of charge conferred to toner through frictional electrification with the surface holding the electrostatic latent image; an electric resistance value of a toner carrier; an effective length of the toner carrier; an effective area of the toner carrier; an amount of adhering toner on the surface holding the electrostatic latent image as a result of development; a moving rate of the surface holding the electrostatic latent image; an amount of adhering toner on the surface of the toner carrier; and a speed ratio between the surface of the toner carrier and the surface holding the electrostatic latent image.
However, in any of the conventional dielectric transfer rollers transfer conditions have not been considered that exert influence on a transfer state during the transfer process, including: a thickness and dielectric constant of a photosensitive layer of the photosensitive body; a thickness, dielectric constant, and amount of adhesion per unit area of a toner layer formed on the photosensitive body; a thickness, dielectric constant, and specific resistance of a recording medium; a thickness and dielectric constant of a dielectric layer of the transfer roller; a thickness, dielectric constant, and specific resistance of a resistance layer of the transfer roller; a potential and exposure saturation potential of a non-exposure region on the photosensitive body before entering into a transfer portion; a relative charge of toner before entering into the transfer portion; a nip width of the transfer portion; a peripheral speed of the photosensitive body; etc.
Neither JP-A 1-230079 nor JP-A 3-87759 supra discloses an equation indicating the transfer conditions using each of the foregoing parameters.
It is therefore an object of the invention to provide a transfer apparatus equipped with a dielectric transfer roller for controlling scattering of toner and enhancing the transfer efficiency during the transfer process by setting transfer conditions adequately in the transfer apparatus, there by making it possible to obtain a high-quality image.
The invention provides a transfer apparatus for transferring toner carried on a photosensitive body, comprising:
a transfer roller including a cylindrical conductive substrate, and a resistance layer and a dielectric layer which are formed on a surface of the cylindrical conductive substrate in this order; and
a photosensitive body including a conductive substrate and a photosensitive layer formed thereon,
the toner carried on the photosensitive body being transferred to a recording medium by applying a transfer voltage to the transfer roller in a state that the recording medium is nipped in a transfer portion having a predetermined nip width between the transfer roller and the photosensitive body,
wherein a relation expressed by the following Equation (1) is established:
Ccxc2x7Vv/(xcfx81txc2x7dtxc2x7m)xe2x89xa70.9xe2x80x83xe2x80x83(1)
wherein
Vv=xe2x88x92V1+Vb+Vt
Cc=1/(1/Cp+2/Ct+rm+1/Cr3)
Cp=∈p/dp
Ct=2xc2x7∈t/dt
rm=xcfx81mxc2x7dm
Cm=∈m/dm
Cr1=∈1/d1
C1=1/(1/Cp+2/Ct+1/Cr1)
Ex=(W/vp)/{rmxc2x7(C1+Cm)}
Cr3=Cr1xc2x7{1xe2x88x92C1/(C1+Cm)}xc2x7exp(xe2x88x92Ex)
xcfx81t=qpmxc2x7m/dt
Vt=xcfx81txc2x7dt/Ct,
wherein dp (m) and ⊂p (F/m) are a thickness and dielectric constant of the photosensitive layer of the photosensitive body, respectively, dt (m), ∈t (F/m) and m (kg/m2) are thickness, dielectric constant and amount of adhesion per unit area of a toner layer formed on the photosensitive body, respectively, dm (m), ∈m (F/m) and xcfx81m (xcexa9xc2x7m) are thickness, dielectric constant, specific constant of the recording medium, respectively, d1 (m) and ∈1 (F/m) are thickness and dielectric constant of the dielectric layer of the transfer roller, respectively, d2 (m), ∈2 (F/m) and xcfx81r (xcexa9xc2x7m) are thickness, dielectric constant and specific resistance of the resistance layer of the transfer roller, respectively, V1 (V) is potential of an image region on the photosensitive body before entering into the transfer portion, qpm (C/kg) is relative charge of the toner before entering into the transfer portion, Vb (V) is transfer voltage applied to the transfer roller, W (m) is nip width of the transfer portion, and vp (m/s) is peripheral speed of the photosensitive body.
In the invention, the electrostatic capacity Cp of the photosensitive layer; the electrostatic capacity Ct of the toner layer; the electrostatic capacity Cc of the transfer portion found from the resistance value rm of the recording medium and the electrostatic capacity Cr3 of the transfer roller relative to the recording medium; the potential V1 of the image region on the photosensitive body before entering into the transfer portion; the voltage Vv of the transfer portion found from the transfer voltage Vb applied to the transfer roller and the voltage Vt of the toner layer before entering into the transfer portion; the amount of charge per unit thickness pt of the toner layer; and the amount of adhesion m of the toner layer are set so as to satisfy the relation expressed by Equation (1). The relation with Ccxc2x7Vvxc2x7(xcfx81txc2x7dtxc2x7m) determines the substantial transfer efficiency that indicates a ratio of an amount of toner carried on the photosensitive body before entering into the transfer portion with respect to an amount of toner transferred onto the recording medium having passed through the transfer portion, and the scattering of toner is reduced when the transfer efficiency reaches or nearly reaches its maximum value. Hence, by setting the electrostatic capacity Cc of the transfer portion, the voltage Vv of the transfer portion, an amount of charge per unit thickness pt of the toner layer, and the amount of adhesion m of the toner layer so as to satisfy the relation Ccxc2x7Vv/(xcfx81txc2x7dtxc2x7m)xe2x89xa70.9, it is possible to maintain the transfer efficiency at 90% or above, there by making it possible to prevent the scattering of toner at the transfer portion.
According to the invention, by setting the electrostatic capacity Cp of the photosensitive layer; the electrostatic capacity Ct of the toner layer; the electrostatic capacity Cc of the transfer portion found from the resistance value rm of the recording medium and the electrostatic capacity Cr3 of the transfer roller relative to the recording medium; the surface potential V1 of the image region on the photosensitive body before entering into the transfer portion, the voltage Vv of the transfer portion found from the transfer voltage Vb applied to the transfer roller and the voltage Vt of the toner layer before entering into the transfer portion; the amount of charge per unit thickness xcfx81t of the toner layer; and the amount of adhesion m of the toner layer so as to satisfy the relation Ccxc2x7Vv/(xcfx81txc2x7dtxc2x7m)xe2x89xa70.9, it is possible to maintain the transfer efficiency at 90% or above and upgrade the image quality by preventing the scattering of toner at the transfer portion.